Non-contact optical thermometry is a rapidly growing field with applications in remote-sensing of hostile or corrosive environments, medical imaging, environmental studies and industrial process monitoring. The current best practice in remote temperature sensing involves the use of cryogenic infrared radiometers.
Infrared radiometers are passive sensors operating at wavelengths near 10 μm that measure the “brightness temperature” of the infrared radiation emitted by a radiating body in the environment The temperature sensitivity of an infrared radiometer is determined by its ability to resolve small changes in the radiant emission against background noise. Using sensitive cryogenic quantum well infrared photodetectors (QWIP), temperature sensitivity can be better than 0.02 C.
If the radiating body is a perfect blackbody (an object with 100% emissivity), the brightness temperature is equal to the physical temperature of the radiating body. However, the brightness temperature is less than the physical temperature for an object with an emissivity below 100%. Thus a determination of the physical temperature of a radiating body requires an estimation of the emissivity of the radiator. One consequence of the dependence of the temperature uncertainty on errors in the estimated emissivity is that a 10% inaccuracy in the estimate of the emissivity can give rise to errors of the order of a few percent in the inferred temperature (in degrees Kelvin). In addition, the inferred temperature can be substantially in error if the radiating source does not fill the field of view of the measurement system. The likelihood of such errors has meant that, in many applications, methods of temperature measurement which rely on the spectral information are now preferred over infrared radiometers.
One spectral technique is two-colour pyrometry. Using this technique, the temperature of a radiating body is inferred by measuring the ratio of the source radiation intensity at two independent wavelengths and applying Planck's radiation law, which states that the blackbody spectral radiance is a universal single parameter distribution governed by the temperature T of the radiating source. Planck's law is usually represented by the relationship       H    ⁡          (              v        ;        T            )        =                    2        ⁢        h        ⁢                                  ⁢                  v          3                            c        2              ⁢          1                        exp          ⁡                      (                          hv              /              kT                        )                          -        1            
Integrated over wavelength, the total power P radiated by a surface of area A and emissivity ∈ at temperature T is given by the relationshipP=∈AσT4where σ is the Stefan-Boltzmann constant. In general, the emissivity ∈(ν,T) is dependent on both wavelength and temperature.
In two colour pyrometry, the ratio of the power radiated from a body at two selected, narrow wavelength bands is measured. This approach obviates the need for knowledge of the emissivity or its temperature variation. However, either (a) the radiating body must be grey (that is, its emissivity is less than 1, and is independent of wavelength), in which case ∈(ν)=∈, or (b) the spectral dependence of ∈ must be known. As well as having a reduced sensitivity to emissivity, ratio pyrometers have the benefit of being insensitive to obscuration of the field of view due, for example, to dust, smoke, obstruction, or lens contamination.
Another technique that relies on being able to treat the radiation source as a greybody, in the sense that the spectral variation of the emissivity is unimportant in the region of interest, is Fourier-transform infrared (FTIR) spectroscopy. Fourier transform infiared spectrometers are usually configured as Michelson interferometers in which the translation of one of the mirrors (often piezoelectrically actuated) produces an interference pattern that is registered by a detector or detector array.
It was reported, recently, by P. C. Dufour, N. L. Rowell and A. L. Steele, in their paper in Applied Optics, volume 37, 1998, page 5923, that although spectral techniques are not as sensitive to small temperature changes as radiometers, FIIR spectrometers are now achieving sub-degree temperature resolution.
However, current FTIR spectroscopy does have some disadvantages. For example, for optimum performance, FTIR spectrometers must be carefully aligned. They can also be bulky and sensitive to external noise, and require a computer for Fourier inversion of the interferogram and display of the spectrum. They are not readily configurable as imaging devices; they have limited field-of-view; and they usually require optical path length monitoring using a suitable fixed wavelength source such as a laser.
A recently developed spectral instrument is the so-called MOSS (modulated optical solid-state) spectrometer, which monitors the complex coherence (fringe visibility and phase) of an isolated spectral line at one or more optical delays. This instrument has been used for visible light Doppler imaging of high temperature plasmas in the H-1 heliac. The MOSS spectrometer is described in the specification of International patent application No. PCT/AU98/00560, which is WIPO Publication No. WO 99/04229. It is also featured in the paper by J. Howard, C. Michael, F. Glass and A. Cheetham in Review of Scientific Instruments, volume 72, 2001, page 888. More information about the MOSS system, which is useful in high-resolution studies of spectral lineshape, can be found at the web address htt://rsphyhsse.anu.edu.au/prl/MOSS.html.